1. Field of the Invention
This invention relates to beam expanders, and more particularly to apparatus and methods for expanding and pointing a beam in a desired direction.
2. Description of the Prior Art
Aircraft borne laser radar systems have been developed which are employed as a navigational tool for terrain avoidance. These systems determine the actual range from the aircraft to ground, compare the actual range with the desired range obtained from a flight log, and adjust the path of the aircraft accordingly.
Laser beams in the far infrared regions are normally used for this purpose. The beam is typically generated with a relatively small diameter, perhaps 2-3 cm., in an orientation normal to and centered upon an exit window. The cross-sectional area of the beam must be expanded prior to transmission through the window, typically by a factor of about 20, and the beam axis must be tilted so that it exits the window at a positive angle to the normal. This prevents a "narcissus" effect, or a scattering of energy back into the beam from the window. The entire beam processing apparatus is rotated in continuous steps of 120.degree. each, enabling the beam to scan the terrain and obtain a range for each 120.degree. increment. The received echo signal at each increment has a 90.degree. polarization shift relative to the output beam, and is separated from the output beam to determine the range.
The beam expansion apparatus must be afocal, that is, the output beam produced by the expansion must be collimated as is the smaller input beam. It is generally desirable that the expansion apparatus employ reflective rather than refractive elements, because refractive elements produce more beam scattering and hence a lower signal-to-noise ratio and reduced range. Also, refractive elements have to be adjusted to a specific wavelength, whereas reflective elements are wavelength independent within the band of interest. Reflective elements also generally exhibit lower adsorption and lower sensitivity to temperature changes than do refractive elements.
There is a limit to the rate at which the beam can be expanded, since too rapid an expansion will produce beam aberrations that can prevent proper recollimation. Accordingly, it is necessary to reserve a minimum distance over which the beam is expanded in order to maintain geometric quality. Focusing the beam should be avoided to prevent gaseous molecular breakdown that might occur at a focus due to the high laser energy.
Unfortunately, only limited space is available in an aircraft terrain radar system, and it is therefore particularly desirable that the beam expansion apparatus be as compact as possible. Prior art optical systems such as the Dall Kirkham and Mersenne designs are optically satisfactory for laser applications, but are too bulky for practical use in installations requiring compactness and in geometries in which the input beam is normal to and centered upon an exit beam window. In practice, prior beam expanders for this application have had to use refractive elements, with the deficiencies noted above, in order to fit within the required small volumes.
An example of a prior art reflective beam expander is illustrated in FIG. 1. The system includes a convex secondary mirror 2 which receives a small diameter laser beam 4 and reflects the beam onto a concave primary mirror 6. Due to the convexity of secondary mirror 2, the beam angularly diverges in transit from secondary mirror 2 to primary mirror 6; the primary mirror is positioned such that the beam reaches it when the beam is at the desired output cross-section. The beam is collimated by primary mirror 6 and reflected along an axis 8 parallel to the input beam axis. A plano mirror 10 positioned in the beam path just below the level of secondary mirror 2 reflects the expanded beam onto another plano mirror 12, which points the beam along a desired output axis 14. Mirrors 10 and 12 do not alter the collimation of the output beam, but serve merely to orient it so that it passes through an output window 16 at a desired offset angle to the window. This offset angle determines the beam's line of sight.
The expanded beam axis 8 from primary mirror 6 extends through and normal to the center of exit window 16. In order to sweep the beam through 360.degree. rotations, plano mirrors 10 and 12 are contained in a rotatable drum or barrel, indicated by dashed line 18. The barrel rotates about axis 8, thereby sweeping the beam through a 360.degree. rotation for each turn of the barrel.
The beam expansion system illustrated in FIG. 1 expands the beam to the desired size, and adequately maintains its geometric quality. However, the volume occupied by the equipment is excessive for the desired laser radar application.
Another prior art system which is somewhat more compact, and which allows the small diameter input beam to be aligned with the exit window, is illustrated in FIG. 2. A pair of plano mirrors 20 and 22 redirect the input beam to a convex secondary mirror 24 located near the exit window but out of sight of the exit beam. Secondary mirror 24 reflects the mirror towards a concave primary mirror 26, and causes it to angularly diverge so that is reaches its desired cross-section at primary mirror 26. In this respect the distance between the secondary and primary mirrors of the two systems illustrated in FIGS. 1 and 2 are the same. Primary mirror 26, which is located near the top of a rotatable barrel 28, recollimates the beam and points it along a desired output axis 30 which intersects the center of the window. The FIG. 2 apparatus is somewhat less bulky than that of FIG. 1, but is still considerably larger than desirable.
Another approach employing reflective elements is illustrated in FIG. 3. The height of the beam expansion apparatus above the exit window is less in this system than in the designs shown in FIGS. 1 and 2, but its horizontal extent is greater. The small diameter input beam 32 is reflected at right angles by three plano mirrors 34, 36 and 38 to a convex secondary mirror 40 at the lower right hand corner of the system. Secondary mirror 40 expands and directs the beam onto a concave primary mirror 42 such that the beam is at its desired cross-section when it reaches the primary mirror. For this purpose the distance between the secondary and primary mirrors is the same as in the two previously described systems. The primary mirror 42 collimates and redirects the beam along a generally horizontal axis 44 to a large plano mirror 46, which points the beam out exit window 16 along the desired output axis 48. The entire system is included within a rotatable drum 50 that rotates about the coincident axis of input beam 32 and the exit window. The system of FIG. 3 extends a lesser distance away from the exit window than do the two previously described systems, but it rotates through a slightly greater volume than does the system of FIG. 2. Accordingly, it is still too bulky.
A refractive beam expander which achieves the desired small volume is illustrated in FIG. 4. The input beam 52 is directed through an expansion lens 54, which causes the beam to expand at substantially the same rate as in the previously described systems. The axis of input beam 52 is normal to and extends through the center of exit window 16. The expanding beam is redirected by plano mirrors 56 and 58 so that it is pointed along the desired output axis 60 relative to exit window 16. A primary collimating lens 62 is placed in the beam path just above the exit window to collimate the beam prior to exit. The system is enclosed within a rotatable drum 64.
The volume swept by the refractive system of FIG. 4 as it rotates is small enough for practical use in an airborne laser radar system. Compared with the volumes swept by rotation of the previously described reflective systems, with the swept volume of the refractive FIG. 4 system assigned a unit volume of 1.0, the swept volumes of the FIGS. 1, 2 and 3 systems are approximately 4.6, 2.5 and 2.8, respectively. However, the refractive system of FIG. 4 suffers from the scattering, adsorption, bandwidth and temperature sensitivity problems mentioned previously.